This study focuses on the cell interaction which takes place during the in vivo and in vitro generation of cytotoxic lymphocytes (CLC). In vivo generation of CLC is accomplished by sensitizing mice with allogeneic ascitic tumors. In vitro generation of CLC is accomplished in the mixed lymphocyte culture. The presence of CLC is determined with the in vitro Cr51-cytotoxic assay. After establishing which tissues play a major role in cytotoxic activity and after determining the peak of cytotoxic activity the tissues are treated, before cytotoxic assay, with specific anti-macrophage-, anti-B cell- and anti-T cell serum. The role of these cells in the cytotoxic reaction is thus established.